Method of making a casting mold



United States Patent Office 3,349,830 METHUD OF MAKING A CASTING MOLDHans Schneider, Winterthur, Switzerland, assiguor to Sulzer BrothersLimited, 'Winterthur, Switzerland, :1 Swiss company No Drawing. FiledNov. 20, 1964, Ser. No. 412,861 Claims priority, applicationSwitzerland, Nov. 21, 1963, 14,305/ 63 8 Claims. (Cl. 164-43) Theinvention relates to a method of making casting molds by means ofexpendable patterns for investment casting. In this process, the patternused to make the mold is destroyed after the mold has been made, theactual casting cavity remaining in the mold. Thus patterns of wax orthermoplastics are destroyed, for example by being melted out of themold, any pattern residues burning when the mold is subsequently fired.It has also already been proposed to dissolve the patterns out of themold by means of a solvent, for example organic solvent vapors beingused for wax patterns and water for urea patterns.

As compared with the melting method, the method in which the patternsare destroyed by being dissolved out of the mold has the advantage thatthere is no damage due to different thermal expansion of the patternmaterial and the mold material. On the other hand, the solvent used todissolve the patterns out of the mold may occasionally enter intoundesirable reactions with the binder in the molding material used formaking the mold. This is the case, for example, where aqueous solventsare used in conjunction with molding materials contain ing aconventional silicate compound as binding agent. To obviate thisdisadvantage, it was necessary to use a different solvent, but thisrestricted the selection of possible pattern materials quite apart fromthe possibly higher price of organic solvents as compared with cheapaqueous solvents.

Nor was it possible heretofore to use one of the conventional aqueousbinders in the molding material in conjunction with a pattern made froma water-soluble material. For example, the use of urea patternsprecludes the use of waterglass-like binders or of colloidal aqueoussilica as binder unless the mold is made waterproof by complicatedadditional treatment.

The invention enables the said disadvantages to be eliminated and alsoensures a rational economic production process in the manufacture ofinvestment casting molds. The method according to the invention ischaracterized in that the pattern is molded in a molding materialcontaining a refractory granular molding substance and a substanceactive as a preliminary binder, and that the pattern in the resultantmold is at least partially destroyed by being melted out, burned ordissolved out, and that then the mold is impregnated with a liquid whichcontains or constitutes a material which can act as a secondary binderor be subsequently converted into the same.

In the method according to the invention the actual binder for the molddoes not come into contact with the solvent for the pattern. Thus, inparticular, solvents and pattern materials and binders can be used whichin the previous method would enter into harmful reaction with oneanother. Itis also possible to use cheap aqueous solvents in conjunctionwith the well-tried silicatecontaining binders, provided, of course,that the pattern can be destroyed by an aqueous solvent, as is the case,for

example, with urea patterns, which are particularly advantageous becauseof their crystalline structure. The substance active as the preliminarybinder is intended mainly only to give the mold obtained by means of thepattern a strength sufficient for handling. It is unim- 3,349,830Patented Oct. 31, 1967 portant whether the preliminary binder isdestroyed during further treatment of the mold-for example, drying andfiring. Therefore, for the purposes of this invention, any binder, aswell known in the art, may be used as the preliminary binder so long asit imparts a sufficient binding of the finely divided refractorymaterial for handling purposes.

The preliminary binder is advantageously a synthetic resin, for examplean epoxy resin which is dissolved in an organic solvent and to which, ifrequired, a substance can be added to accelerate curing. The preliminarybinder may alternatively be a self-curing oily binder, for example ofthe conventional kernel oil type used in foundries. The oils involvedhave conjugate double-bonds.

The method according to the invention is very suitable for forming themold by repeated clipping of the pattern into a pasty mold material, thepreliminary binder giving improved strength properties to the variousmold layers, particularly if drying is carried out after each dippingoperation. The liquid used to impregnate the mold may advantageously bea solution or suspension of the binder. The substance contained in theimpregnating liquid may be converted by a special treatment to an activebinder form after the mold has been impregnated. Such a substance would,for the purposes of this invention, be considered a binder. For example,a silica ester can be used, in which case this substance is introducedinto the mold and then may be hydrolyzed in order to convert it to theform active as a secondary binder. The substance used in theimpregnating liquid may advantageously be a metallic salt from which themetallic hydroxide active as binder is precipitated under the action ofan acid. The metallic salt may be a salt of one of the following metals:e.g. titanium, aluminum, zirconium, or silicon. An aqueous solution ofphosphoric or boric acid may be used as a secondary binder.

A particularly advantageous embodiment of the invention is characterizedin that Where an organic binder is used the mold is subjected to afiring treatment before being impregnated with the liquid. This givesbetter binding of the completed mold by the secondary binder. Withoutsuch firing treatment prior to impregnation with the liquid, the organicpreliminary binder would coat an appreciable part of the surface of thefine-grain refractory substances contained in the molding material, andthus block the access of binder liquid to the mold surface parts duringthe subsequent impregnation of the mold. If a firing treatment iscarried out to convert the preliminary binder into a skeleton-likeframework consisting of carbonized hinder, the porosity of the mold isincreased while the access of binders to the fine-grain refractorysubstance is facilitated, and this greatly improves the strength of thecompleted mold after firing. In such conditions the preliminary bindercarbonized by the firing treatment gives the mold sufiicient strengthduring handling for the subsequent impregnation with the liquid used forthe introduction of the secondary binder;

The firing treatment may advantageously be performed in a reducing orinert atmosphere; this prevents any burning of the preliminary binder.The same effect can be' obtained if the firing treatment is carried outwith the exclusion of air or in air at a pressure of less than 1 Torr.Finally, it is advisable to carry out the firing treatment attemperatures in the range from 200 to 900 C. Higher or lowertemperatures may be used depending upon the particular materials beingemployed.

The invention will be described in detail with reference to thefollowing examples, for illustrative purposes.

Example I 12 kg. of finely ground mullit'e-(Al O (SiO -are mixed with asolution of 0.5 kg. of polystyrene (molecular weight approximately20,000 to 40,000) in 5 litres of benzene. This solution of athermoplastic is used as the preliminary binder and forms a pasty moldmaterial with the refractory mullite.

A pattern of the required casting, for example the runner of a turbinetogether with those parts which form the subsequent downgates, is dippedinto the pasty molding material and the thin mold layer adhering to thesurface of the pattern is advantageously dried for minutes byirradiation with infrared lamps. This process is repeated six to tentimes, so that a shell-like mold built up from the corresponding numberof layers forms around the pattern. This mold element forms thesubsequent casting mold and its wall thickness may, for example, be 5mm. The coating formed after each dipping operation is advantageouslydried. The polystyrene solution gives the mold element sufficientstrength for handling.

The pattern is then destroyed by the action of a liquid solvent. We willassume that the pattern has been made from a urea material. This patternmaterial is watersoluble, and the mold element together with the patterncontained therein is therefore placed in a water bath. The water canpass to the surface of the pattern through the mold element pores anddissolves the pattern. The pattern can be completely dissolved by meansof the solvent, or else just partially so that the pattern residue isdestroyed on subsequent firing of the mold element.

The mold element from which the pattern has been completely or partiallyeliminated is placed in methyl silicate (50% SiO for a period of abouttwo minutes. The methyl silicate impregnates the mold so that the poresbetween the refractory mullite particles are filled by liquid methylsilicate. The mold is then exposed to a saturated water vaporatmosphere. The water particles coming into contact with the methylsilicate effect hydrolysis and hence splitting of colloidal silica andliberation of alcohol. The colloidal silica, or silica hydrogel, is theactual secondary binder which on the subsequent firing of the moldelement gives the latter the strength required for the castingoperation. For firing, the mold is heated for four hours at about 800 C.in an oven. After removal, the mold element is ready for pouring as acasting mold. The casting obtained is distinguished by high precisionand very good surface quality. Although a temperature of 800 C. is used,lower temperatures, or higher temperatures, such as 1200 C. or higher,may be employed.

In some cases it is advisable to use an acid of suitable concentration,for example, 1% hydrochloric acid, as a catalyst for hydrolysis of themethyl silicate. This acid can be sprayed on the mold element. Insteadof a water vapor atmosphere, water may be sprayed onto the mold element,the Water having previously been provided with an addition of acid.

Example II 12 kg. of finely ground sillimanite are mixed with 8 kg. ofChinese wood oil to form a paste. This oil has a conjugate double-bondand is conventionally used as a core binder in foundry practice.

As in Example I, this molding material is used to make a mold elementand the pattern is then destroyed in one of the above-described ways.The mold element from which the pattern has been completely or partiallyeliminated is then placed in an aqueous solution of sodium titanate(12%) for at least 15 minutes. The mold element is thus impregnated withthe aqueous solution which to a varying degree fills in the pores whichare formed in the molding material during the drying of the latter.

The impregnated mold element is then placed in a carbon dioxideatmosphere, for example, such gas being blown onto the mold. Titanicacid and sodium carbonate form in the resultant chemical reaction. Thetitanic acid forms the actual secondary binder. The casting mold readyfor casting is obtained by firing at 900 the mold element treated inthis way. The mold is distinguished by very good strength and castingproperties.

Example III 1000 grams of finely ground quartz sand are mixed with asolution of 400 grams of epoxy resin in cc. of acetone to form a moldingmaterial for the production of the mold element containing thepreliminary binder. The casting pattern is repeatedly dipped into thismolding material as in Example I so as to form a shell-like mold elementfrom which the pattern is then removed in the appropriate way. It isadvantageous for the molding material used for the mold to be given asuitable hardener and an accelerator for the epoxy resin; conventionalproducts may be used for this purpose.

The mold element is then impregnated with a 10% aqueous solution ofsodium silicate. The mold is then treated with gaseous CO as in ExampleII, colloidal silica being precipitated as the actual secondary binder.

Example IV The procedure is the same as in Example III, but the sodiumsilicate is replaced by an 18% aqueous solution of sodium aluminate. Bythe action of carbon dioxide gas this is converted to Al(OH) which actsas a binder on firing of the mold element.

Example V A mold element obtained from water-soluble urea by means of apattern as in Examples I and II is impregnated with colloidal silica(18% SiO and then dried. Silica precipitates at a degree of dryingcorresponding to about 40% of SiO and then acts as the actual binderduring firing of the mold, while the preliminary organic binder isdestroyed.

Example VI A mold element obtained in accordance with Examples I or IIis impregnated with an aqueous solution of phosphoric acid (60%) whichconstitutes the secondary binder. After firing (900 C.) the mold hasvery good strength properties and casting cavity surface quality.Instead of phosphoric acid, an aqueous solution of boric acid (3%) canbe used.

Example VII 12 kg. of finely ground mullite-(Al O (SiO are mixed with asolution of 3.5 kg. of polystyrene (molecular weight about 20,000 to40,000) in 5 litres of benzene. This solution of a thermoplastic servesas the preliminary binder and forms a pasty molding material with therefractory mullite.

A pattern of the required casting, for example the runner of a turbinetogether with those parts which form the subsequent dovvngates, isdipped into the pasty molding material and the thin mold layer adheringto the surface of the pattern is advantageously dried for 15 minutes byirradiation with infrared lamps. This process is repeated six to tentimes, so that a shell-like mold built up from the corresponding numberof layers forms around the pattern. This mold element forms thesubsequent casting mold and its wall thickness may, for example, be 5mm. The coating formed after each dipping operation is advantageouslydried. The polystyrene solution gives the mold element sufficientstrength for handling.

The pattern is then destroyed by the action of a liquid solvent. We willassume that the pattern has been made from a urea material. This patternmaterial is watersoluble and the mold element together with the patterncontained therein is therefore placed in a water bath. The water canpass to the surface of the pattern through the mold element pores anddissolves the pattern. The pattern can be completely dissolved by meansof the solvent, or else just partially so that the pattern residue isdestroyed on subsequent firing of the mold element.

The mold element from which the pattern has been completely or partlyremoved is then placed in an O n and fired at a temperature of about 500C. for six hours in a reducing atmosphere. A hydrogen atmosphere is usedfor this purpose. This treatment causes the polystyrene used aspreliminary binder to carbonize and form a skeleton-like lattice whichconsists mainly of coke-like residues. The strength of the mold elementafter this firing treatment is quite sufficient for further handlinguntil the secondary binder comes into operation. During this firstfiring treatment, any pattern residues can melt and provided the moldelement is suitably stored, can flow out through the subsequent ingate.

The mold element obtained in this way is then placed in methyl silicate(50% SiO for a period of about two minutes. The methyl silicateimpregnates the mold so that the pores between the refractory mulliteparticles are filled by liquid methyl silicate. The mold is then exposedto a saturated water vapor atmosphere. The water particles coming intocontact with the methyl silicate effect hydrolysis and hence splittingof colloidal silica and liberation of alcohol. The colloidal silica isthe actual secondary binder which on the subsequent firing of the moldelement gives the latter the strength required for the castingoperation. For firing, the mold is heated for four hours at about 800 C.in an oven. After removal, the mold element is ready for pouring as acasting mold. The casting obtained is distinguished by high precisionand very good surface quality.

As stated in connection with Example I, it is again in some casesadvisable to use an acid of suitable concentration, for example, 1%hydrochloric acid, as a catalyst for hydrolysis of the methyl silicate.This acid can be sprayed on the mold element. Instead of a Water vaporatmosphere, water may be sprayed onto the mold element, the water havingpreviously been provided with an addition of acid.

Example VIII 12 kg. of finely ground sillimanite are mixed with 8 kg. ofChinese wood oil to form a paste. This oil has a conjugate double'bondand is conventionally used in foundries a a core binder.

This molding material is used as in Example VII to make a mold elementand the pattern is then destroyed in one of the said ways. The mold fromwhich the pattern has been completely or partly eliminated is then firedfor four hours at a temperature of 650 C. in an inert nitrogenatmosphere so that the preliminary binder is converted to a binderskeleton consisting mainly of coke residues.

The mold element bound in this Way is then impregnated with colloidalsilica (18% SiO and then dried. Silica precipitates at a degree ofdrying corresponding to about 40% of SiO and then on the subsequentsecond firing of the mold acts as the actual binder, while theskeleton-like structure formed by the preliminary binder is completelydestroyed by burning.

Example IX To produce the mold element provided with the preliminarybinder 1000 grams of finely ground quartz sand are mixed with a solutionof 400 grams of epoxy resin in 100 cc. of acetone to form a moldingmaterial. The pattern of the casting is repeatedly dipped into thismolding material as in Example VII so as to form a shell-like moldelement from which the pattern is removed approximately. A suitablehardener and an accelerator for condensation of the epoxy resin areadvantageously added to the molding material used to make the mold andthey may be conventional products.

The mold element is then subjected to a first firing treatment at apressure of less than 1 Torr in a vacuum furnace at temperatures ofabout 800 C. for a period of about 2 hours.

The mold element is then impregnated with a 10% aqueous solution ofsodium silicate and then treated with gaseous CO this gas being blownonto the mold, for example.

The resultant chemical reaction causes silica to precipitate as theactual secondary binder and becomes active during the second firingtreatment at temperatures of about 900 C.

The invention is not limited to the exemplified embodiments described.The mold element may be produced in some way other than by dipping thesame into a pasty molding material, for example, by the use of flasks.In such cases a pattern of the required casting together with the partsforming the downgates and runner is backed with the patsy moldingmaterial in a flask and the pattern is destroyed preferably by beingmelted out or burnt. The mold element in the flask is then impregnatedwith the liquid which contains, for example, a substance active as thesecondary binder, For this purpose the entire flask can be placed in anappropriate bath or else the liquid can be introduced into the moldelements through the casting cavity. The flask is then removed from thebath and/ or the excess liquid is discharged. The flask with theimpregnated mold element is then dried and fired. The mold element inthe flask can undergo a first firing treatment in accordance withExample VII after the introduction of the pattern, the preliminarybinder being converted during this treatment to a binder frameworkconsisting of coke residues. After firing, the flask can be placed in abath containing the substance active as the secondary binder. The flaskwith the impregnated mold element is then conventionally dried andfired. The firing treatment for carbonization of the preliminary bindermay also be carried out with the exclusion of air; the firingtemperatures are generally between 200 and 900 C.

Molding materials or pattern materials other than those mentioned in theexamples may also be used. Impregnation of the mold element with theliquid may be carried out by spraying the liquid onto the mold element.Finally, the substance introduced into the mold element by impregnationwith the liquid need not necessarily constitute the actual binder as inExample VI; the substance may be such that it is not converted into itsactive binder form until a suitable further treatment has been carriedout after its introduction, for example, by a physical treatment or achemical reaction.

The preliminary binders which may be used according to the invention maybe any of the well-known binders customarily used in producing foundrymolds, such as polystyrene, epoxy resins, such as epoxy resins on thebasis of 2,2-bis-(4-oxyphenyl)-propane (Example III) or on the basis ofdihydroxynaphthalene (Example IX). Suitable solvents for these resinsinclude benzene, ketones and esters, e.g. acetone or acetylbutylate.Hardeners, respectively curing agents which may advantageously be usedwith the epoxy resins are well-known and include triethanolamine andn-amino ethylpiperazine. A suitable accelerator for the epoxy resinsincludes such a well-known accelerator as furfuryl alcohol.

The various oils which may be used as preliminary binders, and whichcontain conjugate double bonds, include the Well-known binders such askernel oils, especially Chinese wood oil and linseed-oil.

The secondary binders which may be used according to the invention aremany of the well-known binders such as colloidal silica which may beprepared by the hydrolysis of a silica ester such as methyl silicate,ethyl silicate, propylsilicate, etc. (or by the destabilization of anysuitable aqueous silicate solution). These esters are advantageouslyhydrolyzed in the presence of acids such as hydrochloric acid, sulfuricacid or phosphoric acid. Other binders which may be used includephosphoric acid or boric acid, as well as the reaction product of sodiumtitanate and carbon dioxide or the reaction product of sodium aluminateplus carbon dioxide.

The refractory materials which may be used according to this inventioninclude any of the Well-known refractory materials employed in precisioncasting, such as zirconium powder, china clays, mullite, sillimanite,quartz, fused quartz and olivine.

I claim:

1. The process of producing a casting mold which comprises forming anadmixture of a finely-divided refractory material and a preliminary lowtemperature binder capable of being carbonized in response to heat,contacting said admixture with a pattern material to form a moldedadmixture, heating the resulting molded admixture to a temperaturesufficiently high to at least partially carbonize said preliminarybinder, impregnating the molded admixture with a high temperaturesecondary binder, and heating the impregnated molded admixture to atemperature sufiiciently high to bind the secondary binder with thefinely-divided refractory material to form a casting mold.

2. The process of claim 1 in which the molded admixture is heated in aninert atmosphere.

3. The process of claim 1 in which the molded admixture is heated in areducing atmosphere.

4. The process of claim 1 in which the molded admixture is heated in anatmosphere which is free from oxygen.

5. The process of producing a casting mold which comprises forming anadmixture of a finely-divided refractory material and a resin,contacting the admixture with a pattern material to form a moldedadmixture, heating the molded admixture to a temperature suflicientlyhigh and in a suitable atmosphere to carbonize the resin, impregnatingthe molded admixture with colloidal silica, and heating the impregnatedmolded admixture to a temperature sufificiently high to bind thecolloidal silica with the finely-divided refractory material to form acasting mold.

6. The process of claim 5 in which the molded admixture is heated to atemperature between about 200 C. and 900 C.

7. The process of claim 5 in which the resin is a polystyrene resin.

8. The process of claim 5 in which the resin is an epoxy resin.

References Cited UNITED STATES PATENTS 2,790,218 4/1957 Kohl et al l6482,817,886 12/1957 Tobler et al. l6443 X 2 ,820,265 1/1958 Kohl et al164-36 1 2,886,869 5/1959 Webb et al. l648 2,930,089 3/1960 Emblem et a1l6416 FOREIGN PATENTS 797,514 7/1958 Great Britain.

I. SPENCER OVERHOLSER, Primary Examiner.

MARCUS U. LYONS, Examiner.

1. THE PROCESS OF PRODUCING A CASTING MOLD WHICH COMPRISES FORMING ANADMIXTURE OF A FINELY-DIVIDED REFRACTORY MATERIAL AND A PRELIMINARY LOWTEMPERATURE BINDER CAPABLE OF BEING CARBONIZED IN RESPONSE TO HEAT,CONTACTING SAID ADMIXTURE WITH A PATTERN MATERIAL TO FORM A MOLDEDADMIXTURE, HEATING THE RESULTING MOLDED ADMIXTURE TO A TEMPERATURESUFFICIENTLY HIGH TO AT LEAST PARTIALLY CARBONIZE SAID PRELIMINARYBINDER, IMPREGNATING THE MOLDED ADMIXTURE WITH A HIGH TEMPERATURESECONDARY BINDER, AND HEATING THE IMPREGNATED MOLDED ADMIXTURE TO ATEMPERATURE SUFFICIENTLY HIGH TO BIND THE SECONDARY BINDER WITH THEFINELY-DIVIDED REFRACTORY MATERIAL TO FORM A CASTING MOLD.